It is a known objective to reduce the thickness of ribbons so as to conserve space and to achieve more printing from a ribbon of a given size. When the ribbon is housed in a cartridge which is discarded after ribbon use, as is now common, overall ribbon cost is significantly reduced when a thinner ribbon permits increased lengths of ribbon to be contained in such a cartridge. Thus, reducing substrate thickness has been a known and important design objective for printer ribbons.
A prior thin film ribbon has a polyethylene substrate which is stretched during extrusion to a typical thickness of between 0.4 and 0.55 mil (approximately 0.001016 cm to 0.001397 cm). No such ribbon is commercially produced having thickness less than 0.35 mil (approximately 0.000889 cm). Thinner polyethylene sheets are difficult to produce, but, more fundamentally, for thinner polyethylene sheets degradation is observed in both print coverage and eradication by lift-off correction. Production of the very thin polyethylene substrate includes a high degree of molten stretching, which affects the surface conditions or surface energy of the film. This apparently has detrimental effects on the interface between the substrate and the ink layer it carries. Also, the increased orientation decreases the ability of the film to emboss under impact, thereby allowing less of the impact energy to be transferred to the ink in the pattern of the printing element.
This invention employs a polymer blend of polyethylene and isotactic polypropylene as the substrate material, with stretching on a chill roll after extrusion as was standard with the polyethylene, but to a much reduced thickness. The resulting ribbon satisfies commercial standards for both printing and lift-off correction and achieves a significant reduction in costs since it is much thinner. This may be used to achieve an acceptable ribbon having an 0.23 mil substrate, which is thinner than known ribbons. This may also be used for substrates in the conventional range of thicknesses to achieve a ribbon of increased ink transfer, which is particularly suitable for printing on rough papers.
Blends of polyethylene and isotactic polypropylene are broadly known, as illustrated by the following known references, but their use as a thin substrate in the transfer medium art is apparently broadly novel. These references are as follows: U.S. Pat. Nos. 4,634,739 to Vassilatos; 4,045,515 to Isaka et al; 3,965,229 to Driscoll; and 3,952,073 to Isaka et al. It is commonly known that polypropylene has a lower surface energy than polyethylene.